JPH04152607A - magnetic field generator - Google Patents

Abstract

PURPOSE:To concentrate magnetic flux effectively in the vicinity of the center of an air gap, and a high strength magnetic field is generated without increasing magnet material by a method wherein the center part of a pole piece is composed of soft magnetic material, and its circumferential part is composed of the material having the permeability lower than the soft magnetic material. CONSTITUTION:A magnetic circuit is composed of a pair of opposing permanent magnet part 1 on which an air gap 5 is formed, and a pair of magnetic field adjusting pole piece 2, which are fixed to the permanent magnet part 1, is magnetically coupled by a tabular yoke 3 and a support pole 4. The center part 21 of the pole piece is composed of an electromagnetic soft steel plate and the circumferential part 22 is composed of SS steel having the permeability lower than that of the electromagnetic soft steel plate which constitute the center part 21. Also, the upper and the lower permanent magnet parts 1 are composed of eight permanent magnet constituent bodies having almost trapezoidal cross-section. By effectively concentrating the magnetic flux coming from the permanent magnet in the vicinity of the air gap center, the intensity of magnetic field in the air gap center can be increased, a high strength uniform magnetic field can be generated in the vicinity of the air gap center using the same quantity as before, and the device can be manufactured by the technique easier than that heretofore in use.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention is directed to a high-strength and This invention relates to a magnetic field generator that generates a highly accurate and uniform static magnetic field.

[Prior art] There are three methods for generating magnetic fields in MHI devices: permanent magnet method, normal conductive magnet method, and superconducting magnet method. Among these, the permanent magnet method involves consumption of electric power and helium. It is the most economical because there is no Furthermore, in recent years, with the advent of rare earth magnets with strong magnetic force and improvements in the performance of signal detection devices and imaging technology, the potential for permanent magnet MRI devices has increased.

In an MHI device, the strength and uniformity of the static magnetic field affect the quality of the captured image, so a high-intensity, uniform magnetic field with an accuracy of 10-' or less is required near the center of the gap into which the subject is inserted.

In order to meet this strict condition in conventional permanent magnet type magnetic field generators, as disclosed in Japanese Patent Application Laid-Open No. 62-256416, the opposing surfaces of a pair of upper and lower permanent magnet portions made of rare earth magnets are placed against the air gap. They were arranged to form a concave curved surface, generating a high-intensity and highly uniform magnetic field near the center of the gap. Or, JP-A-63-241905
As disclosed in , a part of the pole piece has a laminated structure of magnetic plates laminated in a direction substantially perpendicular to the side surface of the pole piece,
A high-intensity and highly uniform magnetic field was generated near the center of the gap.

[Problems to be Solved by the Invention] However, as in the prior art described above, forming a permanent magnet in a concave curved shape or forming a magnetic plate in a laminated structure is difficult to manufacture and has a negative impact on assembly accuracy. There is a big possibility.

Therefore, an object of the present invention is to create a magnetic circuit that can generate a uniform magnetic field of high intensity near the center of the gap using the same amount of magnet material as conventional techniques, and that is easier to manufacture than conventional techniques. It is to propose.

"Means for Solving the Problems" The magnetic field generating device of the present invention magnetically couples a pair of permanent magnet sections facing each other with a gap formed therebetween and a pair of pole pieces fixed to the permanent magnet sections using a yoke and a support. In the magnetic field generating device that generates a magnetic field in the air gap with this structure, the center part of the pole piece is made of a soft magnetic material, and the soft magnetic material is a material having higher magnetic permeability than the material forming the peripheral part. It is characterized by

[Example] FIG. 1 is an explanatory diagram showing a magnetic circuit used in a magnetic field generating device in an example of the present invention. The magnetic circuit has a structure in which a pair of permanent magnet parts 1 facing each other with a gap 5 formed therein, and a pair of magnetic field adjustment pole pieces 2 fixed to the permanent magnet parts 1 are magnetically coupled by a plate-like yoke 3 and a column 4. The center part 21 of the pole piece is made of electromagnetic mild steel plate, and the peripheral part 22 is made of SS having a lower magnetic permeability than the electromagnetic mild steel plate constituting the center part 21.
composed of copper. The magnetic properties of each material are the first
As shown in the table. Further, the upper and lower permanent magnet sections 1 are each composed of eight permanent magnet members each having a substantially trapezoidal cross section. In addition, in order to avoid an increase in the weight of the magnetic circuit, the permanent magnet used in the magnetic field generator of the present invention has a maximum energy product BH□8 of 25 megagauss oversted (
Nd-Fe-B system or Pr
A rare earth magnet such as -Fe-B is desirable.

The above structure makes it possible to increase the magnetic field strength at the center of the gap by effectively concentrating the magnetic flux from the permanent magnet near the center of the gap.

In the magnetic circuit described above, the basic composition is Pr17 atomic %.
, Fe76.5 at%, B5 at%, Cu1.5 at%
A rare earth magnet manufactured by hot/rolling process,
The maximum energy product is 26゜2 MG OeO is 1
．． The center of the gap when using 4 tons, setting the effective gap length L to 500M, and changing the proportion of the electromagnetic mild steel plate in the entire pole piece, that is, the ratio D2/DI of the diameter of the electromagnetic mild steel plate and the diameter of the pole piece. Figure 2 shows the results of measuring the magnetic flux density. From this figure, D2/D
It can be seen that the optimum value of + is around 0°5 to 0.6, and the central magnetic field strength of the air gap 5 is higher than that when the pole piece 2 is molded as one piece.

[Effects of the Invention] As described above, according to the present invention, the center part of the pole piece is made of a soft magnetic material, and the peripheral part is made of a material with lower magnetic permeability than the soft magnetic material in the center part. , it was possible to effectively concentrate magnetic flux near the center of the air gap, and it became possible to generate a higher intensity magnetic field without increasing the amount of magnet material.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the basic structure of a magnetic circuit of a magnetic field generating device in an embodiment of the present invention. FIG. 2 is a diagram showing the relationship between the diameter ratio D 2 /D + of the electromagnetic mild steel plate constituting the center portion of the pole piece and the diameter of the pole piece, and the strength of the central magnetic field. 1... Permanent magnet part, 2... Pole piece, 21...
・Pole piece center part, 22... Pole piece peripheral part, 3... Yoke, 4...
・Protrusion, 5... Support, 6... Gap, Figure 2

Claims (1)

[Claims] A magnetic field generating device in which a pair of permanent magnet parts facing each other with a gap formed therein and a pair of pole pieces fixed to the permanent magnet parts are magnetically coupled by a yoke and a column, and a magnetic field is generated in the gap with this structure. A magnetic field generating device characterized in that a central portion of the pole piece is made of a soft magnetic material, and the soft magnetic material is a material having a higher magnetic permeability than a material constituting the peripheral portion.